1. Field of the Invention
The present invention relates to a television broadcast receiving apparatus, and particularly to a television broadcast receiving apparatus capable of receiving analog broadcasting.
2. Description of the Background Art
In recent years, the shift of television broadcasting from analog broadcasting to digital broadcasting is underway worldwide. The timing of termination of analog broadcasting, however, differs between countries. Globally, the shift from analog broadcasting to digital broadcasting is expected to complete by around 2015. Therefore, at the present time, manufacturers of a television broadcast receiving apparatus manufacture and sell a receiver which allows for watching both analog broadcasting and digital broadcasting.
FIG. 4 is a block diagram showing a configuration of a conventional television broadcast receiving apparatus 500 capable of receiving analog broadcasting. In FIG. 4, television broadcast receiving apparatus 500 includes a tuning unit 200, a demodulating unit 600 and a decode and output unit 400. In the following, the operation of television broadcast receiving apparatus 500 will be described.
Tuning unit 200 has an antenna terminal 201 to which an analog broadcast signal is fed. A bandpass filter 202, an RF (Radio Frequency) amplifier 203 and a bandpass filter 205 select and amplify a desired wave channel signal of the inputted analog broadcast signal. RF amplifier 203 is a variable gain amplifier and its gain is controlled such that a suitable input signal level to a mixer 206 is obtained. A control signal for controlling the gain of RF amplifier 203 is generated by an RF AGC (Automatic Gain Control) detecting part (DET) 204 connected to an output node of bandpass filter 205.
The selected and amplified desired wave channel signal above is mixed at mixer 206 with a signal resulted from frequency dividing of a local oscillation signal of a local oscillating part 208 by a frequency dividing part (1/N) 207, and passed through a bandpass filter 209 to be turned into an IF (Intermediate Frequency) signal. The IF signal is amplified by an IF amplifier 210 such that the signal has a signal input level suitable for demodulating unit 600.
The IF signal outputted from tuning unit 200 is inputted to demodulating unit 600. In demodulating unit 600, first at a PLL (Phase Locked Loop) part 601 the phase of the IF signal is regulated such that the phase of the inputted IF signal and the phase of a carrier signal generated within PLL part 601 are in agreement. The IF signal phase regulated by PLL part 601 is supplied in parallel to a picture filter 602 and a sound filter 603.
The IF signal passed through picture filter 602 is demodulated at a video detecting part (DET) 604 into a video signal of the analog broadcast signal and transmitted via a low-pass filter (LPF) 606 to decode and output unit 400. On the other hand, the IF signal passed through sound filter 603 is demodulated at a sound intermediate frequency (SIF) detecting part (DET) 605 into a sound intermediate frequency signal, and thereafter detected by an FM (Frequency Modulation) detecting part 608 to be converted into an audio signal, and transmitted to decode and output unit 400.
It is noted that the above-described IF amplifier 210 is a variable gain amplifier, and a control signal for controlling its gain is generated by an IFAGC detecting part (DET) 211 connected to an output node of picture filter 602.
At decode and output unit 400, the video signal outputted from demodulating unit 600 is divided into a color signal and a luminance signal and converted into an RGB signal, before being displayed as a picture at a display part. The audio signal outputted from demodulating unit 600 is passed through a bandpass filter 800 before being outputted as a sound from a speaker within decode and output unit 400.
Meanwhile, as described above, the gain of IF amplifier 210 is controlled to achieve a signal level suitable for demodulating unit 600, by a control signal from IFAGC detecting part 211 based on the output signal level of picture filter 602. This results in that when an input signal to IF amplifier 210 enters a no-signal state, an output signal from picture filter 602 also enters a no-signal state, IFAGC detecting part 211 sets the gain of IF amplifier 210 at the maximum value, and IF amplifier 210 amplifies only a noise. The noise amplified by IF amplifier 210 is transmitted to sound intermediate frequency detecting part 605, which causes sound intermediate frequency detecting part 605 to output a very large noise. There has been a problem that detection of this noise by FM detecting part 608 turns the noise into a sound noise, which is outputted from a speaker of an television broadcast receiving apparatus as an abnormal sound offensive to the ear.
Countermeasures have been taken against this abnormal sound, such as imposing a special band limitation using filter 800 described in the operational description given above and muting an audio signal when it is determined at a synchronization determining circuit that synchronization is not established by a vertical synchronization signal of a video signal.
The countermeasures described above also cause some inconvenience. Specifically, although addition of filter 800 against the abnormal sound has achieved reduction of the abnormal sound due to the noise in the IF signal and provided a certain effect, increase of mounted parts in number and use of an expensive filter because of steep frequency characteristics have inhibited achieving less expensive equipment. Further, addition of a filter could not completely eliminate the abnormal sound.
In addition, as for reception of analog broadcasting, even when a smaller input signal to a receiver causes lower picture sensitivity that does not meet a practical level for watching, sound sensitivity may remain at its sufficiently practical level. The countermeasure as described above of muting a sound in response to loss of synchronization of a vertical synchronization signal at decode and output unit 400, however, has a drawback of causing no sound output even in a state in which a smaller input signal to the receiver causes somewhat disturbed synchronization of a picture, which results in lower practical sound sensitivity.
Japanese Patent Laying-Open No. 06-030358 (Patent Document 1) discloses a television sound demodulating apparatus. The apparatus is provided with means for detecting locking of respective PLL parts of a video signal synchronization system and a sound system for suppressing an unpleasant crunching noise due to variations in locking time of a PLL part of a digital sound demodulator, means for generating a mute control signal based on both of these lock signals and an output enabling signal, and means for controlling on/off of muting of an analog audio signal, a digital audio signal and a bit stream signal using the mute control signal. In turning power on/off or switching input signal, the apparatus always sets the optimum muting period, independently of variations in locking time of the PLL parts of the picture/sound systems.
Further, Japanese Patent Laying-Open No. 08-102686 (Patent Document 2) discloses a digital audio data reproducing apparatus. In this apparatus, in order to reduce a noise which is offensive to the ear and generated when clock synchronization cannot be established, when data synchronization cannot be established, and when data error is detected, a PLL circuit, a synchronization detection circuit and an error correction circuit supply clock unlock signal, a loss of synchronization signal and a data error detection signal, respectively, to a mute signal weighting circuit, a mute signal corresponding to each of the signals is outputted to an output control circuit, and the output control circuit control output of audio data according to the mute signal.
In Patent Document 1, however, the picture system and the sound system are provided with respective PLL parts, and the sound mute control signal is generated based on the lock signals of the respective PLL parts and the output enabling signal for controlling the timing and period of outputting an audio signal. Therefore, the circuit configuration and control of the signals is rather complex and not suitable for achieving less expensive apparatus. In addition, it cannot be said that consideration is given to a sound mute operation in the case of increased or decreased input signal level to the apparatus.
In Patent Document 2, when the carrier to noise ratio (C/N) degrades due to conditions of an input signal level to the apparatus, causing the PLL part to lose phase synchronization and to enter a clock unlocked state, a mute signal is immediately supplied from the mute signal weighting circuit to the output control circuit to mute audio data and set output of audio data zero. In this case, although noise reduction can be effectively achieved, setting output of audio data zero depending on the condition of reception makes occurrence of no-sound state unavoidable, and therefore, an abrupt break in a sound is experienced in the vicinity of the reception limit.